1. Field of the Invention
The present invention relates generally to a non-intrusive sensor within a composite structure. More particularly, the present invention relates to such a sensor for sensing waste matter in an aircraft's waste tank system.
2. Related Art
The vast majority of large containers used to house liquids utilize some form of equipment and technology to determine the level of the fluid contained within. It is quite common to see a mechanical float or some form of an electronic sensor being used to determine the fluid level within a water storage container or fuel tank of a car. The same holds true for the pressurized, composite waste tank of an aircraft. In fact, the sensing system is highly critical within a waste tank in order to notify the pilot and/or flight crew of an aircraft when a tank may be full. The sensing equipment aides in preventing overboard discharge of waste matter which could cause mechanical failures and contamination to other downstream equipment.
Due to the environment contained within the waste tank system, sensing of the waste matter held within tends to be a difficult and often problematic process. The majority of sensing systems used for other liquids has assorted shortcomings within the waste tank environment.
Sensor systems that float on the surface of the fluids will easily foul up with paper particles and waste matter contained within a waste tank.
Load cell sensor systems that measure variations in the weight of the waste tank have difficulties in measuring accurately. This is primarily due to hard point connections all around the tank, like pipes and mechanical attachments that can adversely load or support the weight of the tank.
Pressure sensor systems that measure the difference in hydrostatic pressure (weight) of the fluid can be inaccurate in measuring the volume of fluid in a waste tank in that the contents of the tank (paper products, liquid, and solid matter) vary in density, and thus weight.
Top-down ultrasonic sensor systems in which an ultrasonic signal travels down from the top of tank, bounces off the surface of the fluid, and returns to the sensor receiver does not work well in a waste tank environment. The violent inbound flow of matter from the toilets, movement of the aircraft during flight and turbulence, and cyclic pressure variations of the vacuum waste tank system causes foaming and sloshing of the liquid and waste matter which in turn creates inconsistencies in the sensing signal.
The most common method of determining the full level of a waste tank currently is through point level sensors in which a sensor intrudes through the wall of a tank and senses the full level through direct contact sensing, ultrasonic, or by other means. Although a highly accurate measurement, the intrusive method raises concerns due to the reduction in structural integrity of the tank from the sensor intrusion through the tank wall, the possibility of leakage from the penetration (creating a biohazard), and the possibility of fouling of the sensor from the waste matter.
Existing nonintrusive sensing technology sends a signal (acoustic, ultrasonic, etc.) through the wall of a container and returns it to a receiver. This method does not have to account for variation in density of the waste matter held within the tank. However, this method has proven to be problematic due to the composite laminate commonly used to fabricate the waste tank. The voids (air pockets), variations in resin and fiber volumes, and thickness differences in the composite laminate of the tank wall create large inconsistencies in the sensing signal, and have thus proven to be unreliable.